The present invention relates to fine particles composing a developer used in an electrophotographic method, an electrostatic recording method, an electrostatic printing method, a magnetic recording method and the like.
The electrographic method is conducted as follows in general; electrical charges are uniformly applied to a photoconductive layer and exposed to lights in accordance with images, whereby electrostatic latent images are formed. Then the electrostatic latent images are developed by toner particles charged in a polarity opposite to that of the charges applied to the electrostatic latent images. The developed toner particles are transferred onto a transfer sheet, and fixed.
Toners used in such a development of electrostatic latent images are, in general, composed of fine particles such as colorants, magnetic particles and the like dispersed in binder resin.
However, if those fine particles are not uniformly dispersed in the binder resin, various kinds of harmful effects appear.
For example, if the coloring matters are not uniformly dispersed in the toner particles, electrical charges are not uniform among the toner particles and thus the toners which lack charges are liable to be produced to bring about scattering of toners, whereby fogged obscure images are formed. In addition, the scattering of toners causes also pollution of an inside of copying machine.
In order to improve dispersibility of colorants, the treatments of the colorants with metallic soaps, titanium coupling agents and silane coupling agents have been disclosed (for example Japanese Patent Application Laid-Open No. Sho 53-17735, Japanese Patent Application Laid-Open No. Sho 57-17737, Japanese Patent Application Laid-Open No. Sho 58-7648, Japanese Patent application Laid-Open No. Hei 1-145663 and the like) but in the present invention, surfaces of fine particles are coated with a polyolefinic resin. Accordingly, the present invention is quite different from that of the cited references in constitution. Furthermore, in order to prevent an off-set phenomenon, various kinds of wax have been added to toners (for example, Japanese Patent Application Laid-Open No. Sho 49-65232, Japanese Patent Application Laid-Open No. Sho 49-65231, Japanese Patent Application Laid-Open No. Sho 50-27546, Japanese Patent Application Laid-Open No. Sho 61-80261 and the like). However, if the wax is not uniformly dispersed in a binder resin of toner particles, a problem occurs in that a photosensitive member and the like are filmed with toners.
Besides, in order to prevent toners from scattering, magnetic particles are added to the toners. Since the magnetic particles are inorganic substances and inferior in dispersibility in the binder resin, a large amount of magnetic particles crop out and a content of magnetic particles is not uniform among toner particles. As a result, a transportability and a developability of toners become different and charging properties become unstable under humid conditions. The density of copied images becomes nonuniform.
In addition, in order to prevent blocking of toners and to improve fluidity, fine particles such as hydrophobic silica, alumina, titanium and the like have been added to toners (for example Japanese Patent Application Laid-Open No. Sho 46-12144, Japanese Patent Application Laid-Open No. Sho 48-47346, Japanese Patent Application Laid-Open No. Hei 1-142558, Japanese Patent Application Laid-Open No. Sho 55-120041, Japanese Patent Application Laid-Open No. Sho 60-186862 and the like), but when those fine particles are inferior in humidity resistance or weak in change of humidity, the functions of preventing a blocking and the fluidity are hindered and the quantity of charges on the toners is not uniform and thus serious problem occur in the scattering of toners and various kinds of harmful effects resulting from the scattering of toners.
On the other hand, various kinds of carriers for use in the electrophotographic developer have been proposed and repeatedly improved.
For example, electrically conductive carriers such as iron oxide particles are superior in solid developability but inferior in gradation properties, while coated insulating carriers are superior in gradation properties but inferior in solid developability.
In order to improve these disadvantages, binder-type carriers with fine particles such as ferrite particles, magnetic particles and the like dispersed in a binder resin have been proposed and practically used.
However, the magnetic particles are inorganic substances, while the binder resin is an organic substance. They are originally different to each other, so that an adherence of the magnetic particles to the binder resin is poor and thus the magnetic particles are liable to be broken away from the binder resin due to the collision of the carriers themselves or with the toners during the time when the developer is being stirred. As a result, the developer is not constant and unstable, in chargeability, and thus the quantity of charges on the toners is bad and not uniform, whereby problems are liable to occur in scattering of toners, fogging, pollution by toners and the like.
In addition, the magnetic particles give a bad influence upon the environmental stability and their chargeability is changed with a change in humidity and thus problems occur in the poor chargeability of toners, the scattering of toners, the fogging and the like.
As to an improvement of the environmental stability in respect of humidity, binder carriers with magnetic particles dispersed in a binder resin containing nonpolar resins such as polypropylenes and polyethylenes as an indispensable component have been proposed (for example Japanese Patent Application Laid-Open No. Sho 63-58360, Japanese Patent Application Laid-Open No. Sho 63-182667, Japanese Patent Application Laid-Open No. Hei 1-204071 and the like) but problems have occurred in that a bond strength of the magnetic particles to the polypropylenes or polyethylenes is weak and thus the magnetic particles are broken away from the binder resin during the mixing and stirring in a developing device to change the quantity of charges, whereby there arise the same problems as above described. In addition, since the polypropylene resins or the polyethylene resins used have a low molecular weight within a wax range, the developer aggregates and thus a problem occurs also in that the developer and the like are inferior useful life time.